PORTLAND, Ore. Superlattices may hold the key to commercializing resistive random access memories, according to researchers at the Missouri University of Science and Technology. Using superlattices of magnetite and zinc ferrite, the researchers report are two new materials that they claim may lead to faster, smaller and more energy efficient RRAMs.
Superlattices are atomically thin alternating layers of different materials whose resulting semiconducting properties are strikingly different from those of either material alone. After much experimentation, professor Jay Switzer and his colleagues at Missouri S&T report two superlattices, one composed of different formulations of magnetite (an iron oxide) and a second superlattice material using different formulations zinc ferrite, each of which can have their resistance switched from high to low, making them a candidate for the next-generation memory chip known as resistive random access memory (RRAM).
"Our original ideas was to make spintronic devices, but we found by accident that the electron transport through the material could be switched, making them a candidate for RRAMs," said Switzer.
RRAMs are being proposed as the non-volatile memory of choice for the 22-nanometer process technology node on the International Technology Roadmap for Semiconductors, below which traditional flash memory bit cells are not expected to scale well. Companies working on RRAM designs include Fujitsu, Sharp and Hewlett Packard (the latter for its memristor technology). Also startup Adesto Technologies Inc., an AMD spinoff, has licensed a programmable metallization cell (PMC) technology for RRAMs from Axon Technologies Corp. which is based on research at Arizona State University. And in Europe, the research institute IMEC is doing independent research on RRAMs with its partners Samsung Electronics Co. Ltd., Hynix Semiconductor inc., Elpida Inc. and Micron Technology Inc.
|Electrodeposited magnetite superlattice could enable small, fast and energy efficient resistive random access memory.|
The superlattice materials reported by Switzer and his colleagues at Missouri S&T alternate two different formulations of magnetite, or in a second material, alternative different formulations of zinc ferrite. After fabrication, the materials exhibited variable resistance depending on the amount of voltage applied. Switzer also found that the resistance of the material could be set at various different levels, allowing for the possibility of multiple bits per cell to be encoded in future RRAMs.
The researchers grow their superlattice in solution using electroplating with two different voltages on a crystalline gold substrate.
"The novelty of our technique is that we are essentially electroplating the superlattice, by just changing the voltage potential to deposit the alternating layers," said Switzer.
For the future, Switzer's group is experimenting with various other formulations, trying to understand the underlying mechanism that make them work, as well as characterizing the length of time memories stored by the materials will remain nonvolatile.
Switzer performed the work with Elizabeth Kulp, a postdoctoral associate at Missouri S&T, along with doctoral candidates there Rakesh Gudavarthy, Guojun Mu, Zhen He, and undergraduate student Andrew Wessel.